Modulated carrier wave telephony system



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% 1:2. ft/(ERSfiEJ MODULATED CARRIER WAVE TELEPHONY SYSTEM Filed March 1, 1938 2 Sheets-Sheet 2 Carrie/- fie uenc y P. P. 56559815) 1 y:

Patented Feb. 20, 1940.

MOD'ULA.'1lE-D' CARRIER WAVE TELEPHONY SYSTEM Peter Pendleton Eokersley, Chelsea, London,

England, assignor to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application March 1, 1938, Serial No. 193,224 InGreat Britain March 1 1937 10 Claims.

The present invention relates to modulated carrier wave telephony systems.

In United Kingdom Specification No. 445,431

there is described a modulate-d carrier-wave communication system in which means is provided for modifying the intensity frequency characteristic of the spectrumcreated by the normal modulation of a carrier wave, in such a way that the carrier frequency and one side-band are substantially fully utilised while, in the other side-band, although the components of frequencies close to the carrier frequency are not greatly attenuated,

the middle and outer parts of that sideband are cut away as much as possible consistent with the retention of sufiicient of the attenuated sideband components to prevent the setting up of audible harmonic distortion.

The reason for this choice of shape for the attenuation characteristic of the filter network by which the asymmetry was achieved is, as explained in the earlier specification, that harmonic distortion must be set up if the amplitude of one sideband component is less than its counterpart created by the same frequency of modulation but that this harmonic distortion decreases as the modulation depth becomes smaller. It

was further pointed out that the spectrum of sound frequencies representing typical intelligence transmitted in broadcasting practice had its maximum intensity in a band of frequencies lying roughly between 250 and 1500cycles/sec. and that at frequencies above 1500 cycles/sec.

the intensity in the spectrum decreased rapidly with increasing frequency.

Consequently audible distortion can be avoided so long as the lower modulation frequency sideband components, which are liable to appear at high amplitude, are sufficiently strongly represented in both sidehands, even though the modulation frequency components above a certainfrequency are greatly attenuated in one sideband. Further investigations have now shown that distortion arises not only due to differences in.

amplitude between corresponding components in the two sidebands, but also to other causes and the results of these investigations will now be briefly discussed.

Representing the frequencies produced by the process of modulation of a carrier with a single modulation frequency by three vectors in the usual way in which one vector represents the carrier frequency and the two others the two sideband frequencies one having an angular velocity equal to that of the carrier plus the angular velocity of the modulation frequency and the other an angular velocity equal to that of the carrier minus the angular velocity of the modulation frequency, it can be shown that unless the resultant of the two sideband vectorsis at all times in phase with the carrier vector there will be distortion of the carrier wave envelope.

It will be clear that the condition of no distortion can only exist when both sideband vectors have the same magnitude and when the algebraic sum of the angles and s between the two sideband vectors and the carrier vector is zero. Thus if the sideband vectors are of unequal magnitude, even if a is equal to e, the locus of the tip of the resultant sideband vector, instead of being coincident with the carrier vector will be an ellipse having its major axis coincident with the carrier vector. i

If the sideband vectors are of equal magnitude but if on is not equal to B, the locus above mentioned will be a straight line inclined, to'the carrier vector at an angle If the sideband vectors are of unequal magnitude and such that on is not equal to ,6, then the locus, will be an ellipse having its major axis inclined with respect to the carrier vector at an angle In all these cases where the resultant sideband vector is not co-angular with the carrier vector harmonic distortion will occur. However, distortion due to this cause is most serious where the depth of modulation is considerable, and

Where the depth of modulation is small, that is for the, higher modulation frequencies, the disto'rtlon may not be audible even though the resultant sideband vector is far from oo-angular with the carrier vector. Moreover it is found that wherethe ratio of the. amplitudes of the sideband components is much less than unity, distortion due to phase shifts is less serious than where the amplitudes are more nearly equal.

It follows therefore that in order to reduce harmonic distortion to a permissible level over the whole modulation spectrum, it is necessary not only to arrange that the filter network which produces the asymmetrical attenuation has a respouse characteristic of such shape that the ratio of sideband amplitudes is only decreased substantially from unity as the depth of modulation decreases with increase of modulation frequency but also to arrange that the filter network is such cause the resultant sideband vector to depart substantially from co-anguiarity with the carrier vector at frequencies at which the depth of modulation. may be considerable.

Although a substantial reduction in the width of the channel required for a given quality of transmission can be effected with the aid of the invention set forth in United Kingdom Specification No. 445,431, nevertheless further reduction is desirable. It has been found, however, that if an attemptbe made to increase the attenuation of the outer sideband components of the attenuated sideband, by using an f -type filter for example, distortion is introduced because the phase characteristic of such a filter is markedly asymmetrical in the frequency region which must necessarily be used for the sideband frequencies associated with at least relatively deep modulation.

It is the object of the present invention to provide means whereby the desired further attenuation of the outer sideband components of one sideband can be achieved without the harmonic distortion being thereby increased beyond permissible limits.

According to the present invention, therefore, in a modulated carrier wave system for the transmission and reception of speech and music, there is provided a method of producing differential attenuation of the two sidebands of a carrier wave (that is to say substantial attenuation of one sideband whilst subjecting the other sideband to relatively little attenuation) according to which the attenuation of components of frequencies within 1500 cycles per second of the carrier frequency of one sideband is made less than 8 d. b., the attenuation of sideband components of the same sideband is arranged to increase, between 1500 and 4000 cycles, by at least 20 d. b., whilst the phase distortion, measured by deviation of the resultant sideband vector from co-angularity with the carrier vector, at least up to 3000 cycles per second from the carrier frequency is made less than Further according to the present invention there is provided, in a modulated carrier wave system for the transmission and reception of speech and music, a circuit arrangement for producing substantial differential attenuation of the two sidebands of a carrier wave, the circuit arrangement having characteristics such that within the total gamut of frequencies between the carrier frequency and the extreme frequency of the attenuated sideband there is a range of frequencies associated with relatively deep modulation over which the phase distortion, measured by deviation of the resultant sideband component vector from co-angularity with the carrier vector, is substantially less, in a corresponding region relatively to the cut-off frequency, than that of an f -type filter section arranged to reach substantially infinite attenuation at the said extreme frequency, whilst there is another range of frequencies further from the carrier frequency than the first named range, and associated with relatively shallow modulation, over which the rate of increase of attenuation with change of frequency, in a direction away from the carrier frequency, is greater than that of a single section band-pass filter, such as the constant k: type of the 3 element type, terminated in its image impedence.

the form of a filter network can be constituted that any phase shifts produced thereby do not to give the desired results will be understood from the following. Filter networks of any one type which are designed to produce substantial differential attenuation of counterpart sideband components also produce phase asymmetry between these components. If to one filter section be added another section of the same type in order to increase the differential attenuation, not only will the attenuations of the two sections be added together but the phase asymmetries produced will also be added together.

The desired result can, however, be achieved by forming the filter network of two sections of diiferent types, one section having, over a range of frequencies at which the depth of modulation may be considerable, a characteristic with a negative fiexure and the other a characteristic with a positive fiexure. These two opposite flexures should be made as nearly as possible equal to one another in magnitude, or at least such that the flexure of the resultant on one side of the carrier frequency is as nearly as possible the mirror image of that on the other side of the carrier frequency at least over the range of frequencies referred to.

According to a feature of the present invention, therefore, the network for producing differential attenuation comprises two filter circuits constituted so as to co-operate in producing the differential attenuation, one of the filter circuits producing substantial phase asymmetry between counterpart sideband components over a range of sideband frequencies corresponding to modulation frequencies at which the depth of modulation is large, and the other of said filter circuits being constituted to reduce said phase asymmetry over the said range of frequencies.

The desired attenuation and phase characteristics may be obtained by employing, in combination, band-pass and band-elimination filters having suitably related attenuation and phase characteristics and cut-ofi frequencies. The terms band-pass and band-elimination are used in this specification in a wide sense to include cases where one cut-off frequency is at infinity or zero. Thus, according to a further feature of the present invention the network for producing the differential attenuation comprises, in combination, band-pass and band-elimination filters, the carrier frequency of the system being arranged to be between one cut-off frequency (the lower cut-off frequency where the lower sideband is to be attenuated) of the band-pass filter and the geometric mean between the two cut-off frequencies thereof, and the opposite cutoff frequency (the upper cut-off frequency in the case above assumed) of the band-elimination filter being arranged to be further from the carrier frequency than the said cut-off frequency (the lower cut-01f frequency in the case above assumed) of the band-pass filter.

The invention will be described with reference to the accompanying drawings, in which Fig. 1 is a curve showing approximately the way in which the percentage modulation MN varies with modulating frequency A in the case where the modulation is in the form of normal speech and music.

Fig. 2 shows a section of a constant is type filter,

Fig. 3 is an attenuation characteristic for a constant k type filter, I

Fig, 4 shows a section of a constant k type filter followed by an f section,

according to the present invention, and

Fig. 9 shows a modification of a part of Fig. 5 or Fig. 8, which may be used according to this invention.

The curve of Fig. 1 was obtained experimentally over long periods of observation and represents approximately the maximum intensities which occur in broadcast speech and music over the sound spectrum. In this curve the modulation depth MN, expressed as a percentage, is plotted as ordinate against the modulating frequency A) plotted as abscissa. I

It will be noted that at about 1000 cycles per second, the curve begins to fall and above about 1500 cycles per second the drop in maximum intensity becomes rapid. i

It is on. account of the existence of conditions represented approximately in Fig. 1 that it is possible to remove a large part of one sideband and nevertheless to avoid objectionable distortion. The distortion met with when one sideband is removed is largely that due to the introduction of a second harmonic. At frequencies at which the maximum intensities are high (up to 1500 cycles per second for example) both sidebands should be strongly represented in order to avoid audible distortion. On the other. hand, at higher frequencies, increasing lack of symmetry between the magnitudes of the sideband vectors and between the angular relations of l the two sideband vectors to the carrier vector can occur without audible distortion taking place because of the decreasing maximum intensity of the higher frequency components.

In Fig. 2 is shown a single section constant is type filter terminated in mid-shunt by a resistance R equal to the image impedance of the filter. Such a filter may be used to produce asymmetry between the two sidebands by locating the carrier frequency In between f1 (the lower cut-off frequency) and fm (the geometric mean between the lower and upper cutoff frequencies) and nearer to f1 than to fm. This will be understood by reference to Fig. 3 which shows the attenuation characteristic of a constant is type filter section. The attenuation a in decibels is plotted as ordinate against frequency, is being the upper cut-off frequency. Thus the generator G with its internal resistance 1' may be regarded as representing the modulated magnifier valves of a wireless transmitter and the aerial and earth may be connected in place of the resistance R, a suitable transformer being interposed if desired. Preferably, however, one or more power magnification stages are interposed between the filter network and the aerial in order that the filter network may be traversed by relatively low power oscillations.

' The effect of the use of the circuit of Fig. 2 is indicated in Figs. 6 and 'Z. In Fig. 6 the attenuation a in decibels is plotted as ordinate against frequency as abscissa, the carrier frequency itself being indicated at it (in the case shown at 25 kilocycles per second). fm is arranged to be about 27.5 and f1 about 24 kilocycles per second.

The attenuation characteristic for the circuit of Fig. 2 is shownat A, this curve being the same as the left hand part of the curve of Fig. 3

kilocycles the behaviour of the filter is not unsatisfactory, below this figure the rate of change of attenuation begins to decrease instead of continuing to increase as is desirable. i

Fig. '7 shows the relation between the algebraic sum 5 of the angles between the sideband vectors and the carrier vector on the one hand and modulating frequency a), on the other hand. That is, it indicates the phase asymmetry produced between counterpart sideband components. Curve A is that for the constant is type filter section of Fig. 2. The distortion represented by this phase asymmetry is not serious because the maximum amplitude is becoming very small at frequencies at which the phase asymmetry becomes large.

The objection to the circuit of Fig. 2 is therefore that its attenuation characteristic (A in Fig. 6) is far from. ideal.

The effects of adding an f section to the circuit of Fig. 2, as shown in Fig. i, are indicated at Bin Figs. 6 and'i. Although the attenuation characteristic below 21 kilocycles per second is evidentlygreatly superior to that of the constant it filter, the phase distortion as shown in Fig. 7 is excessive.

One circuit which may be provided according to the present invention is shown in Fig. 5. It comprises a constant is type band-pass filter section BP coupled by a resistance pad RP to a con stant is type band-elimination section BE. In the circuit shown mid-seriesterminations are used but mid-shunt terminations may of course be employed. It is arranged that theupper cutoff frequency fz of the band-elimination filter BE is somewhat lower than the lower cut-01f frequency )1 of the band-pass section BP and the rrangement is made such that severe attenuation (greater than 5 decibels for example) due to the band-eliinination filter only commences at a frequency (corresponding to a modulation frequency greater than 1500 cycles per second) at which the modulation is relatively shallow.

The characteristics of the circuit of Fig. 5 are shown in Figs. 6 and '7 at C. It will be seen in Fig. 6 that the attenuation characteristic is far better than that of either of the other two circuits of Figs. 2 or 4 (curves A and B) and that nevertheless the phase asymmetry is shown in Fig. 7 to be much less than that of either of the other two circuits. Thus it will be seen from Fig. 7 that here is a range of frequencies from zero to 1500 cycles per second (which have been shown in Fig. 1 to be associated with deep modulation) over which in curve C the deviation of the resultant side band vector from cc-angularity with the carrier vector, and hence the phase distortion, is substantially zero and in any case much less than that of the f type filter, shown at B, arranged so as to reach substantially infinite attenuation at the extreme frequency of the attenuated sideband. Moreover in Fig. 6 it is clear that over a range of frequencies further from the carrier frequency, for example below 22 kilocycles per second, the slopeof curve C and hence the rate of increase of attenuation with change of frequency in a direction away from the carrier frequency, is substantially greater than that shown at A of a single section band-pass filter terminated in its image impedance.

Although in the circuits described the filter network comprises only one section of each type, more than one section of either or of both types may be used if desired.

to a different scale. Although down to about 23 The reason for coupling the two sections BP and BE by means such as the resistance pad RP is that the apparent impedances of the two sections vary very differently from one another over the working frequency range. The resistance pad RP is designed in such a way that each section, looking towards the pad, sees an approximation to its image impedance whilst the value of the shunt element is such as to give adequate coupling.

Instead of a resistance pad, other suitable forms of coupling, such as a thermionic valve stage, may be employed.

The band-pass and band-elimination filter sections of Fig. 5 may be replaced by band-pass and band-elimination filter sections of other suitable types. Moreover, if desired, the band-elimination filter section may be arranged in front of the band-pass filter section.

In using the circuit of Fig. 5, the generator G and its internal impedance 1' may be replaced by a valve stage, such as a modulated magnifier stage, in which a normally modulated carrier wave is present and the asymmetrical output can then be taken from across the output resistance R0. The output can be radiated from an aerial system or impressed upon conductors, as for example in wire broadcasting.

In spite of the fact that the first section BP of the circuit of Fig. 5 is terminated in an approximation to its image impedance, nevertheless the impedance as seen at the terminals of the generator G, r is not a pure resistance and varies with frequency. Certain forms of generator, such as a thermionic valve stage comprising valves arranged as modulated magnifiers, are particularly sensitive to this change of output impedance with frequency.

A feature of the present invention whereby this disadvantage can be reduced is illustrated in Fig. 8. In general, in order to achieve the desired result, the filter network is designed and coupled to the valve stage in such a Way that, viewed from the output of the valve stage, the impedance of the filter, over the working range of frequencies, is much greater than (for example five or more times) the value of a resistance which is arranged in the output of the valve stage. The preferred arrangement for achieving this object is by the use of a transformer of high step-down ratio as shown in Fig. 8, as this usually enables the elements of the filter network to be given convenient values. In Fig. 8, the valves V1 and V2, which may be regarded as modulated magnifiers, are coupled by a transformer T1, which has a large step-down ratio such as :1 say, to the first section BE of the network giving asymmetry, a load resistance R2 being connected as shown. In this case the first section BE is the band-elimination section. This section is terminated in its image impedance by a resistance R1.

The effect of the large transformation ratio is that the effective impedance of the filter network as seen by the valves V1, V2 is very high compared with the resistance R2 so that the variations in image impedance of the network have no appreciable effect upon the valves V1, V2.

In the example of Fig. 8, the section BE is coupled to the band-pass section (not shown) by means of valves V3, V4 instead of by the resistance pad RP of Fig. 5.

The band-pass and band-elimination filter sections may take any suitable form and those shown in the drawings are only examples of suitable arrangements. An alternative form of band-eliminator, namely one of the f type, is shown in Fig. 9.

though the invention has been described as applied to a transmitter, it may be applied, if desired, at a receiver at some point before detection takes place. In this Way reduction of interference from one unwanted signal having an adjacent carrier frequency can be obtained. The circuits of the present invention can usually be operated more satisfactorily at relatively low carrier frequencies, for example 20 or 30 kilocycles per second. Where the carrier frequency to be radiated, or otherwise transmitted, is much higher than this it may be desirable to produce the desired sideband asymmetry at a relatively low frequency and then to increase the frequency by any suitable method of frequency multiplication. For instance the increase in frequency may be produced by known heterodyning methods.

The invention is not limited to the preferred methods and circuits described which involve the passing of a normally modulated carrier wave through two filter networks both assisting in producing the desired attenuation and one compensating partially for the phase distortion produced by the other.

For instance it is within the scope of the present invention to utilize a filter network such as the constant is type terminated in an f type section to produce the required difierential attenuation and to provide additional means for correcting phase distortion produced by the filter network, this means not necessarily affecting to any considerable extent the differential attenuation.

If I refer in the appended claims to a modulated carrier Wave telephony system, I wish this to be understood as referring to the transmission and reception of carrier waves modulated with impulses within the audible range, i. e., of speech and music.

The nomenclature employed in the present specification is that used by T. E. Shea, M. S., in the textbook Transmission Networks and Wave Filters, 1929 edition.

I claim:

1. In a modulated carrier wave telephony system, a circuit arrangement for producing substantial differential attenuation of one sideband of a carrier wave relatively to the other sideband thereof, said circuit arrangement comprising attenuating means for producing, over a range of sideband frequencies associated with relatively shallow modulation, attenuation of said one sideband which increases, proceeding away from the carrier frequency, at a rate greater than that of a single section, constant is type band-pass filter terminated in its image impedance, Whilst producing substantially less attenuation of said one sideband over a range of frequencies associated with relatively deep modulation and means for rendering the phase distortion produced by said attenuating means, measured by deviation of the resultant sideband component vector from 00- angularity with the carrier vector, over a range of sideband frequencies associated with relatively deep modulation, substantially less than that of an f type filter section arranged reach substantially infinite attenuation at the extreme frequency of said one side-band.

2. In a modulated carrier wave telephony system, a filter network connected to influence a modulated carrier wave, for the purpose of pro- 55 duced by said first filter section.

ducing substantial differential attenuation of the two sidebands of the modulated carrier wave, said network comprising two filter circuits constituted so as to cooperate in producing said differential attenuation, one of said filter circuits producing substantial phase asymmetry betweencounterpart sideband components over arange of sideband frequencies corresponding to modulation frequencies at which. the depth of modulation is carrier wave being arranged to be between said upper and lower cut-off frequencies of said bandpass filter circuit and the geometric mean between said two cut-off frequencies thereof, and the opposite cut-off frequency of the bandelimination filter circuit being arranged to be further from the carrier frequency than the one of said cut-off frequencies of said band-pass filter circuit lying onthe same side with respect to the carrier frequency as the said opposite cut-off frequency.

4. A system according to claim 3, wherein said carrier frequency is arranged to be nearer to the said cut-off frequency of the band-pass filter circuit than to the said geometric mean.

.5. Asystem according to claim 3, wherein said band-elimination filter circuit is arranged to produce attenuation greater than 5 decibels only at sideband frequencies of the attenuated sideband differing by more than 1500 cycles from said carrier frequency.

6. In a modulated carrier wave telephony system, a filter network connected to, influence a modulated carrier wave for the purpose of producing substantial differential attenuation of the two sidebands of the carrier wave, said network comprising a first filter section for producing substantial differential attenuation of said two sidebands, said filter section also producing substantial phase asymmetry, between counterpart components of said sidebands and a second filter section for substantially increasing said diifererrtial attenuation and for producing substantial phase asymmetry between said counterpart sideband components which is opposite to that pro- 7. In a modulated carrier wave telephony systeeth, a filter network connected'to influence a modulated carrier wave for the purpose of pro:

ducing substantial differential attenuation'of the two sidebands of the carrier wave, said network comprising a first filter section for producing substantial differential attenuation of said two. sidebands, said filter section also producing sub-. stantial phase asymmetry "between counterpart components of said sidebands, a second filter section for substantially increasing said differential attenuation and for producing substantial phase asymmetry between said counterpart sideband components which is opposite to that produced by said first filter section and, between said fist r and second filter sections, coupling means comprising series and shunt resistance elements.

8. In a'm'odulated carrier wave telephony system, a filter network connected to influence, a modulated carrier wave for the purpose of producing substantial differential attenuation of the two sidebands of the carrier wave, said network comprising a first filter section for producing substantial differential attenuation of said two sidebands,said filter section also producing substantial phase asymmetry between counterpart componentsiof said sidebands, a second filter section for substantially increasing said difierential attenuation and for producing substantial phase asymmetry between said counterpart sideband components which is opposite to that produced by said first filter section and, between said first and second filter sections, coupling means com-' prising a thermionic tube having a grid circuit and an anode circuit, said grid circuit being coupled to said first filter section and said anode circuit being coupled to said second filter section.

9. Modulated carrier wave apparatus comprising a thermionic tube having an input circuit and an output circuit, means for applying to said input circuit a modulated carrier wave, a resistance connected in shunt with said output circuit and coupled to said output circuit a filter.

network for producing substantial differential attenuation of the two sidebands of said carrier wave, said network havingan impedance which,

measured at the point of connection to said out- 1 neighbourhood of said carrier. frequency, is several times the value of said resistance.

10. Modulated carrier wave apparatus compris ,put circuit, over a range of frequencies in the ing a thermionic tube having an input circuit and an output circuit, means for applying to said input circuit a modulated carrier wave, a resistance connected in shunt with said output circuit, a transformer having its primary winding connected to said output circuit, a filter network.

connected to the secondary winding of said trans- .forme'r, for producing. substantial differential attenuation of the twosidebands of said carrier wave, said transformer having a step-downjratio of at' least 5:1 and said network having an im-' pedance which, measured across said primary. winding, over a range of frequencies in the neighbourhood of said carrier frequency, is several times the value of said resistance.

PETER PENDLETON ECKERSLEY. 

